ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1055 



causes a slight loss on the side of the ordinary ray, which intrudes a 

 little more than before. 



Now, taking the Nicol prism as it is ordinarily made, there would 

 be a real gain, if it could, without additional labour or cost, be so cut 

 as either to widen the field (using the same length of prism as before) 

 or to shorten the length 6f the prism (if obtaining the same angular 

 aperture as before). The method of cutting adopted by Hartnack, 

 and that suggested in 1881 by Prof. Thompson, both add to the cost 

 of spar and to the labour of cutting. In Hartnack's construction the 

 width of field is gained partly by employing linseed oil, partly by the 

 device of making the plane of the film lie at right angles to the 

 crystallographic axis of the spar. In Prof. Thompson's construction 

 of 1881 the balsam-film was made to lie in a principal plane of section, 

 whilst the principal axis of vision through the prism was made to lie 

 at right angles to the crystallographic axis. A gain of about 9° in 

 the width of the field over that of a Nicol of the same external form 

 was the result ; being a little less for flattened prisms, a little more 

 for oblique-ended prisms. 



Prof. Thompson has now devised a simple modification of the 

 mode of cutting the Nicol prism, which possesses several of the ad- 

 vantages of these costlier methods of construction, but without adding 

 appreciably to the cost. 



Fig. 225 shows the ordinary Nicol prism as usually cut, the end- 

 faces AB and CD being natural faces of the crystal polished up. 

 The books assert that makers of Nicol prisms cut down these faces, 

 making them still more oblique by 3°, but the author has not found 

 any constructor who does this. The natural angle between the face 

 A B and the arrete AD is about 109°. The crystallographic axis 



Fig. 225. 



•-__ ±.iS^. 



makes about 45" with the end-face A B. The balsam-section is at 

 about 90° to the plane of the end-face. The consequence of this is 

 that there are about 45° between the plane of the balsam-film and the 

 crystallographic axis. This limits the field : those rays which traverse 

 the prism at small angles to the film, and which would traverse the 

 film if the crystallographic axis were at right angles to it (as in the 



